1. Field of the Invention
The present invention relates to a solid electrolytic capacitor. More particularly, it relates to a roll type solid electrolytic capacitor.
2. Description of the Prior Art
In electrolytic capacitors, a film-forming metal capable of forming an insulating oxide film, that is, a so-called valve metal, such as aluminum, tantalum, niobium or titanium, is used as a positive electrode. An oxide film layer is formed on the surface of the valve metal by anodic oxidation or the like and this oxide film layer is utilized as a dielectric material. A negative electrode is arranged on the outside of the oxide film layer with an electrolyte layer interposed therebetween, and thus a capacitor is constructed.
As the electrolyte, there can be mentioned a liquid electrolyte formed by dissolving an inorganic or organic acid or a salt thereof in water or a polar solvent and a solid electrolyte composed of an electroconductive or semiconductive inorganic oxide or organic substance such as manganese dioxide, lead dioxide or a tetracyanoquinodimethane complex salt. A capacitor comprising the latter electrolyte is called a solid electrolytic capacitor.
Various shapes have been proposed for the valve metal positive electrode substrate. A roll form capacitor formed by winding a sheet-form substrate into the form of a roll is advantageous in that the capacitance per unit volume is large. However, in a capacitor of the type where two electrode foils having a paper separator interposed therebetween are wound, such as a conventional electrlytic capacitor comprising a liquid electrolyte or a capacitor comprising a TCNQ salt as the solid electrolyte as disclosed in Japanese Unexamined Patent Publication No. 58-17,609, reduction of the volume is limited because of the special structure thereof.
Furthermore, if a liquid electrolyte or TCNQ salt is used, the electroconductivity is low and generally 10.sup.-1 S.cm.sup.-1 or less, and characteristics of the capacitors, such as the loss coefficient (tan .delta.) and the impedance, are apt to be adversely influenced.